Re: [PATCH] static_call,x86: Robustify trampoline patching

Peter Zijlstra
2021-10-30T18:03:08Z

On Sat, Oct 30, 2021 at 07:19:53PM +0200, Ard Biesheuvel wrote:
> I just realized that arm64 has the exact same problem, which is not
> being addressed by my v5 of the static call support patch.

Yeah, it would.

> As it turns out, the v11 Clang that I have been testing with is broken
> wrt BTI landing pads, and omits them from the jump table entries.
> Clang 12+ adds them properly, which means that both the jump table
> entry and the static call trampoline may start with BTI C + direct
> branch, and we also need additional checks to disambiguate.

I'm not sure, why would the static_call trampoline need a BTI C ? The
whole point of static_call() is to be a direct call, we should never
have an indirect call to the trampoline, that would defeat the whole
purpose.